Peach tree with red leaf spots and dropping leaves

I have a young peach tree I purchased from a nursery, it was growing really fast when I transplanted it to a pot, but suddenly it stopped growing and a side of the branches is color red as well as the leaves, which are also curled to the inside. At first I thought it was nitrogen deficiency so I applied a fruit trees fertilizer which is 15-5-20. What could be happening to the tree?